As an example of a transmission scheme employed in the transmission of mass data such as in terrestrial digital television broadcasting and high speed radio LAN (Local Area Network), the OFDM method which is one type of the multicarrier transmission method is recently attracting attention. According to this OFDM method, data is transferred by arranging a train of symbols of the input data in parallel, and then assigning the data formed of symbols in parallel to a plurality of subcarriers which are orthogonal to each other.
More specifically, the signal transmitted by the OFDM method (referred to as OFDM signal hereinafter) is generated by assigning code data serial-parallel converted to a plurality of subcarriers having frequencies in orthogonal relationship with each other using a modulator, applying an inverse Fourier transform (convert frequency region into time region) on respective code data for conversion into digital modulation waves and then applying parallel-serial conversion on the obtained digital modulated waves. At the demodulator side, the original code data can be reproduced by applying a process opposite to the aforementioned process at the modulator side.
FIG. 43A represents the spectrum of a transmission signal and a reception signal in the modulation method of a single carrier wave (single carrier modulation). In contrast, FIG. 43B represents the spectrum of a transmission signal and a reception signal of the OFDM method.
As shown in FIG. 43A, the transmission signal through the single carrier modulation method is affected by the frequency selective fading caused by variation in the frequency characteristics of the transmission path, whereby the quality of the reception signal is significantly degraded. In contrast, the transmission signal by the OFDM method shown in FIG. 43B is impervious to the influence of frequency selective fading since the band width of each subcarrier is small with respect to variation in the frequency characteristics of the transmission path. Therefore, degradation in the quality of the reception signal can be reduced. Since data is transmitted using a plurality of subcarriers in the OFDM method, there is the advantage that the usage efficiency of frequency is good.
However, in the OFDM method, synchronization between the transmission frequency and the reception frequency will be lost when the Doppler phenomenon occurs in which the frequency band of the transmission signal is shifted or the tuner of the receiver is unstable, resulting in frequency deviation Δf (referred to as frequency offset hereinafter) from the original subcarrier frequency, as shown in FIG. 44. This frequency offset will alter the phase of the reception signal to degrade the decoding ability of the receiver.
Occurrence of such frequency offset in reception signals in the OFDM method that uses a plurality of subcarriers will disrupt the frequency orthogonality between subcarriers. If the received OFDM signal is input to the Fourier transformer of the receiver in such a state, the signal component of a subcarrier adjacent to that subcarrier will appear as an intermodulation component in the output of the Fourier transformer to prevent the original code data from being reproduced properly. This induces the problem that the quality of reproduced data is degraded.
It is to be noted that, as the number of subcarriers in the OFDM method increases, respective subcarriers will be distributed more densely in respective determined bands. Therefore, even a small frequency offset will cause severe interference between adjacent subcarriers. Thus, compensation for a frequency offset is one of the most important issues to be considered in implementing a system.
Conventionally, various approaches have been proposed as to the technique of detecting and compensating for such frequency offset. For example, an approach is disclosed in “Synchronization Scheme of OFDM Systems for High Speed Wireless LAN”, TECHNICAL REPORT OF IEICE, DSP97-165, SAT97-122, RCS97-210 (1998-01) by Takeshi Onizawa et al.
As an example of a conventional transmission and reception system of an OFDM signal, a system that employs a DQPSK (Differential Quadrature Phase Shift Keying) system as a modulation scheme and that carries out delay detection at the receiver side will be described hereinafter with reference to FIGS. 45–51.
First, a structure of a conventional OFDM signal transmitter will be described with reference to FIG. 45. In FIG. 45, the signal line represented by a bold line indicates a complex signal (a signal formed of an in-phase detection axis signal and an orthogonal detection axis signal) whereas the signal line represented by a thin line indicates a real number signal.
As shown in FIG. 45, the former half of the conventional OFDM transmitter includes a serial-parallel converter 1 applying serial-parallel conversion on input information signals, a code modulator 2 applying modulation such as DQPSK on information signals in parallel assigned to respective subcarriers, an inverse discrete Fourier transformer 3 applying an inverse discrete Fourier transform on signals output from code modulator 2, a parallel-serial converter 4 converting the signals output from inverse discrete Fourier transformer 3 into signals in series, and a guard section insert circuit 5 adding a guard section at the beginning of the output signal from parallel-serial converter 4 to generate a data symbol.
The latter half of the conventional OFDM transmitter includes a memory 6 storing the preamble and the start symbol of known symbols added to the beginning of a packet, a switcher 7 switching the preamble, start symbol and data symbol for output according to the switching clock supplied from a controller 1000 that will be described afterwards, a digital quadrature modulator 8 providing the real component and imaginary component of the output from switcher 7 as one signal component, a D/A converter 9 converting the output of digital quadrature modulator 8 into analog data, and a frequency converter 10 converting the frequency of the analog data from D/A converter 9 to transmit an OFDM signal. The OFDM transmitter further includes a controller 1000 formed of a CPU and the like to control the overall operation of the OFDM transmitter.
The signal format of the OFDM signal generated by the above-described OFDM transmitter is formed of known symbols including the preamble and (two) start symbols attached at the beginning of a packet, and a data symbol having inverse discrete Fourier transformed data added with a guard section.
More specifically, the data symbol is generated by copying the signal of a section length Tgi at the latter half of the output (valid symbol section) of inverse discrete Fourier transformer 3 and applying the same ahead (the guard section) of the valid symbol section. This application of a guard section allows robustness to a delayed wave that arrives lagging for a period of time within the guard section length Tgi.
The preamble forming the known symbol is a signal used in the gain adjustment of automatic gain control (AGC), symbol synchronization, and the like. The start symbol forming the known symbol serves to determine the initial phase in carrying out modulation by differential coding, and is a signal including all the subcarriers. The length of each start symbol is equal to the valid symbol section length Tw excluding the guard section from the data symbol.
In the case where N subcarriers with the frequency interval of df are used, the signal amplitude must be sampled N times during the start symbol length Tw(=1/df).
The structure of a conventional OFDM signal receiver will be described here with reference to FIG. 47. In FIG. 47, the signal line represented by a bold line indicates a complex signal whereas the signal line represented by a thin line indicates a real number signal.
Referring to FIG. 47, the former half of the conventional OFDM receiver includes a frequency converter 11 converting the frequency of a reception signal to a predetermined band, an A/D converter 12 converting the output of frequency converter 11 into digital data, a digital orthogonal detector 13 separating the output of A/D converter 12 into a real component and an imaginary component, a frequency offset compensator 14 compensating for a frequency offset and estimating a symbol timing (position), and a symbol clock generator 15 generating a symbol clock based on a symbol position estimate value from frequency offset compensator 14.
The latter half of the conventional OFDM receiver includes a guard section removal circuit 16 removing the guard section from the output of frequency offset compensator 14 according to a guard section removal clock supplied from a controller 2000 that will be described afterwards based on the symbol clock generated from symbol clock generator 15, a serial-parallel converter 17 applying serial-parallel conversion on the output from guard section removal circuit 16, a discrete Fourier transformer (FFT) 18 applying discrete Fourier transform on the output from serial-parallel converter 17, a code determination circuit 19 demodulating the output of discrete Fourier transformer 18, and a parallel-serial converter 20 applying parallel-serial conversion on the output of code determination circuit 19. The OFDM receiver further includes a controller 2000 formed of a CPU and the like that controls the overall operation of the OFDM receiver.
Referring to FIG. 48, digital orthogonal detector 13 of FIG. 47 includes a local oscillator 21 oscillating at a constant frequency, π/2 phase shifter 22 shifting by π/2 the phase of the output signal from local oscillator 21, multipliers 23 and 24 multiplying the output of A/D converter 12 of FIG. 47 by respective outputs of local oscillator 21 and π/2 phase shifter 22, and filters 25 and 26 extracting desired complex signals from respective outputs of multipliers 23 and 24.
Referring to FIG. 49, frequency offset compensator 14 of FIG. 47 includes a delay unit 31 delaying the output signal from digital orthogonal detector 13 (FIG. 47) by a valid symbol section length Tw, a cross correlator 32 computing a cross correlation value between the output of delay unit 31 and the reception signal from digital orthogonal detector 13, and an autocorrelator 33 computing the autocorrelation value of the reception signal from digital orthogonal detector 13.
Frequency offset compensator 14 includes a peak detector 34 detecting the peak position of the cross correlation value independent of the reception signal level by dividing the output of cross correlator 32 by the output of autocorrelator 33, and a symbol synchronization position estimator 35 providing an estimate value of the symbol position from the output (cross correlation value peak position) of peak detector 34.
Frequency offset compensator 14 further includes a rotation angle estimator 36 estimating the rotation angle of the cross correlation value from the output of cross correlator 32 and the output of peak detector 34 (cross correlation value peak position) to output an estimate value of the frequency offset, and a phase rotation circuit 37 providing a signal compensated for a frequency offset by rotating the phase of the reception signal from digital orthogonal detector 13 based on the estimate value of the frequency offset from rotation angle estimator 36.
Referring to FIG. 50, correlators 32 and 33 of FIG. 49 include a delay line 41, a tap 42, and an adder 43 to calculate the correlation value by integrating the first input signal using the tap number obtained from the second input signal.
More specifically, cross correlator 32 receives a reception signal from digital orthogonal detector 13 (FIG. 47) as the first input signal and a delayed version of the reception signal from digital orthogonal detector 13, delayed by Tw at delay unit 31, as the second input signal. A cross correlation value can be obtained by integrating the first input signal over the number of taps M obtained from the second input signal.
Autocorrelator 33 receives in common the reception signal from digital orthogonal detector 13 (FIG. 47) as the first and second input signals. By integrating this reception signal over a number of taps M obtained from the reception signal, an autocorrelation value is obtained. Here, the number of taps M is equal to the number of points (the maximum number of subcarriers determined depending upon the structure of FFT 18) of discrete Fourier transformer (FFT) 18 (FIG. 47).
The operation of frequency offset compensator 14 will be described hereinafter with reference to FIG. 49.
Peak detector 34 of FIG. 49 provides the peak position of a cross correlation value by dividing the output of cross correlator 32 by the output of autocorrelator 33. Based on the detected result of peak detector 34, symbol synchronization position estimator 35 generates a symbol synchronization position estimate value.
Since each correlation value is calculated with a complex number, the rotation angle Δθ with respect to the real axis of the cross correlation value can be estimated from the peak position of the cross correlation value at rotation angle estimator 36, as shown in FIG. 51. Based on this rotation angle Δθ, rotation angle estimator 36 can estimate frequency offset value Δf using the following equation.Δf=Δθ/(2πTw)
Based on frequency offset value Δf estimated by rotation angle estimator 36, phase rotation circuit 37 can compensate for a frequency offset by rotating the phase of the reception signal from digital orthogonal detector 13 (FIG. 47). Since rotation angle Δθ with respect to the real axis of the cross correlation value takes a value from −π to π, the frequency offset in the range of −1/(2Tw) to 1/(2Tw) can be compensated for.
Although the frequency offset is compensated for by rotating the phase of a reception signal using phase rotation circuit 37 in the above-described conventional frequency offset compensator 14, the frequency offset of the reception signal can be compensated for without using phase rotation circuit 37. More specifically, frequency offset value Δf obtained from rotation angle Δθ by rotation angle estimator 36 is applied to the control input not shown of local oscillator 21 in digital orthogonal detector 13 shown in FIG. 48. By controlling variably the oscillating frequency, the frequency offset of the reception signal can be compensated for.
However, the conventional frequency offset compensator employs the delay autocorrelation method using a delayed version of the reception signal as a reference signal. There was a problem that only a frequency offset in the range of −0.5 to +0.5 can be detected and compensated for as to the normalized frequency offset, normalized at the frequency interval of the subcarrier.